Method of Making a Lithographic Printing Plate

ABSTRACT

A method of making a lithographic printing plate includes the steps of: a) providing a lithographic printing plate precursor including (i) a support having a hydrophilic surface or which is provided with a hydrophilic layer, (ii) a coating on the support including a photopolymerizable layer and, optionally, an intermediate layer between the photopolymerizable layer and the support, wherein the photopolymerizable layer includes a polymerizable compound, a polymerization initiator and a binder, b) image-wise exposing the coating in a plate setter, c) heating the precursor in a pre-heating unit within a time period of less than 10 minutes after step (b), d) treating the precursor in a gumming station, including at least one gumming unit, whereby a gum solution is applied to the precursor, thereby removing non-exposed areas of the photopolymerizable layer from the support and gumming the plate in a single step.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 of PCT/EP2006/068303, filed Nov. 9, 2006. Thisapplication claims the benefit of U.S. Provisional Application No.60/741,140, filed Dec. 1, 2005, which is incorporated by referenceherein in its entirety. In addition, this application claims the benefitof European Application No. 05110918.9, filed Nov. 18, 2005, which isalso incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for making a lithographicprinting plate whereby a negative-working photopolymer printing plateprecursor is image-wise exposed, heated in a pre-heating unit within atime period of less than 10 minutes after image-wise exposing, andtreated with a gum solution in a gumming station, whereby the plate isdeveloped and gummed in a single step.

2. Description of the Related Art

In lithographic printing, a so-called printing master such as a printingplate is mounted on a cylinder of the printing press. The master carriesa lithographic image on its surface and a printed copy is obtained byapplying ink to the image and then transferring the ink from the masteronto a receiver material, which is typically paper. In conventional,so-called “wet” lithographic printing, ink as well as an aqueousfountain solution (also called dampening liquid) are supplied to thelithographic image which consists of oleophilic (or hydrophobic, i.e.,ink-accepting, water-repelling) areas as well as hydrophilic (oroleophobic, i.e., water-accepting, ink-repelling) areas. In so-called“driographic” printing, the lithographic image consists of ink-acceptingand ink-abhesive (ink-repelling) areas and during driographic printing,only ink is supplied to the master.

Printing masters are generally obtained by the so-calledcomputer-to-film (CtF) method, wherein various pre-press steps such astypeface selection, scanning, color separation, screening, trapping,layout, and imposition are accomplished digitally and each colorselection is transferred to graphic arts film using an image-setter.After processing, the film can be used as a mask for the exposure of animaging material called a plate precursor and after plate processing, aprinting plate is obtained which can be used as a master. Since about1995, the so-called ‘computer-to-plate’ (CtP) method has gained a lot ofinterest. This method, also called ‘direct-to-plate’, bypasses thecreation of film because the digital document is transferred directly toa printing plate precursor by means of a so-called plate-setter. Aprinting plate precursor for CtP is often called a digital plate.

Digital plates can roughly be divided into three categories: (i) silverplates, which work according to the silver salt diffusion transfermechanism; (ii) photopolymer plates which contain a photopolymerizablecomposition that hardens upon exposure to light, and (iii) thermalplates of which the imaging mechanism is triggered by heat or bylight-to-heat conversion. Thermal plates are mainly sensitized forinfrared lasers emitting at 830 nm or 1064 nm. Photopolymers can besensitized for blue, green, or red light (i.e., wavelength range between450 and 750 nm), for violet light (i.e., wavelength range between 350and 450 nm), or for infrared light (i.e., wavelength range between 750and 1500 nm). Laser sources have been increasingly used to expose aprinting plate precursor which is sensitized to a corresponding laserwavelength. Typically, an Ar laser (488 nm) or a FD-YAG laser (532 nm)can be used for exposing a visible light sensitized photopolymer plate.The wide-scale availability of low cost blue or violet laser diodes,originally developed for data storage by means of DVD, has enabled theproduction of plate-setters operating at shorter wavelength. Morespecifically, semiconductor lasers emitting from 350 to 450 nm have beenachieved using an InGaN material. An infrared laser diode emittingaround 830 nm or a Nd-YAG laser emitting around 1060 nm can also beused.

Typically, a photopolymer plate precursor includes a support, aphotopolymerizable coating, and an overcoat. The photopolymerizablecoating includes a polymerizable compound, a polymerization initiator,and a binder, and the overcoat includes usually a polyvinylalcoholbinder to hinder the penetration of oxygen in the coating. Uponimage-wise exposure, free radicals formed by the initiator are notquenched by the oxygen and can initiate crosslinking and/orpolymerization of the polymerizable compound, resulting in hardening orcuring of the exposed areas. The exposed precursor is usually processedin an alkaline developer having a pH>10, whereby the overcoat and thephotopolymerizable coating at the non-exposed areas are solubilized inthe developer solution. U.S. 2004/0131974 discloses a method for makingsuch a lithographic printing plate whereby, after image-wise exposure,the overcoat is removed with water in a pre-washing step, resulting in areduction of the formation of sludge in the alkaline developing solutionand whereby staining is prevented at the non-image portions.

Currently, most commercial lithographic plates require an additionalgumming process after the exposed plate is developed and before it isput on the press, in order to protect the plate from contamination,e.g., by oxidation, fingerprints, fats, oil or dust, or from damaging,e.g., by scratches during handling of the plate. Such an additionalgumming step is not convenient for the end-user because it is a timeconsuming step and requires an additional gumming station.

WO 02/101 469 discloses a method of processing an imageable elementuseful as an alkaline-developable lithographic printing plate precursorwherein the element is developed and gummed with an aqueous alkalinedeveloping-gumming solution including a water-soluble polyhydroxycompound having a specific structure.

EP 1 342 568 discloses a method for making a heat-sensitive lithographicprinting plate wherein the image-wise heated precursor, including acoating of hydrophobic thermoplastic polymer particles which coalescenceon heating, is developed with a gum solution. A practical preferredembodiment for this type of printing plate was introduced by Agfa underthe tradename Azura.

In U.S. Pat. No. 6,027,857, U.S. Pat. No. 6,171,735, U.S. Pat. No.6,420,089, U.S. Pat. No. 6,071,675, U.S. Pat. No. 6,245,481, U.S. Pat.No. 6,387,595, U.S. Pat. No. 6,482,571, U.S. Pat. No. 6,576,401, andU.S. Pat. No. 6,548,222, a method is disclosed for preparing alithographic printing plate wherein a photopolymer plate, afterimage-wise exposure, is mounted on a press and processed on-press byapplying ink and fountain solution to remove the unexposed areas fromthe support. Also, U.S. 2003/0016577 and U.S. 2004/0013968 disclose amethod wherein a plate including a photopolymerizable layer can beprocessed on-press with fountain solution and ink or with a non-alkalineaqueous developer. An adhesion promoting compound can also added to theprinting plate precursor for improving the developability of theon-press processing and for improving the durability of the plate in theprinting process. Typically, these compounds have an ethylenicallyunsaturated bond and a functional group capable of adsorbing to thesurface of the support. Other compounds and polymers can be used as anadhesion promoting compound. The compound can be present in thephotopolymerizable layer or in an intermediate layer between the supportand the photopolymerizable layer as disclosed in EP 851 299, EP 1 091251, U.S. 2004/0214105, EP 1 491 356, U.S. 2005/0039620, EP 1 495 866,EP 1 500 498, EP 1 520 694, and EP 1 557 262.

A first problem associated with on-press processing of such photopolymerprinting plates is the lack of daylight stability, i.e., the image isnot stable before processing and, therefore, the exposed plate needs tobe processed within a short time after the exposure. However, sinceon-press processing is not possible during a print job, the end-usermust wait until the previous print job has been completed before theexposed plate can be mounted on the press and processed. As a result,the exposure of the plate for the next print job must be delayed untiljust before the completion of the previous print job, so as to avoidthat the unprocessed plate is affected by the ambient light.Alternatively, the exposed plate must be kept under safe-lightconditions, but this again reduces the ease of use and convenience thatare normally associated with, e.g., violet- and infrared-sensitivephotopolymer plates.

A second problem left unsolved in the prior art about on-pressprocessable photopolymer plates is the lack of a visible image betweenexposure and processing. Although it is known to add a colorant to thephotosensitive coating, so as to obtain a visible image after removal ofthe non-exposed areas of the coating by the processing, this does notmake it possible to distinguish an exposed plate from an unexposed plateimmediately after the image-wise exposure, let alone to inspect theimage quality after the exposure, because the visible image is onlyrevealed after the on-press processing. Moreover, on-press processableplates normally do not contain a colorant because the on-press removalof the non-printing areas of the coating may cause contamination of thefountain solution and/or the ink and it may take an unacceptable numberof printed copies before the contamination by the colorant hasdisappeared.

A third problem associated with on-press processing with fountainsolution and ink is an insufficient clean-out of the non-exposed areas.

In WO 2005/111727, a method for making a lithographic printing plate isdisclosed wherein the image-wise exposed precursor is developed with agumming solution.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention provide a method for making a lithographicprinting plate from a photopolymer plate precursor, which is perceivedby the user as a method which does not require a processing step andwherein the exposed plate can be kept in ambient light for an unlimitedtime before being mounted on the press. A preferred embodiment of thepresent invention includes a method having the specific feature that theimage-wise exposed plate precursor is heated in a pre-heating unitwithin a time period of less than 10 minutes after image-wise exposing,and treated with a gum solution in a gumming station including at leastone gumming unit whereby the plate is developed and gummed in a singlestep. Since the plate is developed and gummed, the lithographic imagecan no longer be affected by ambient daylight. On the contrary, furtherexposure to daylight would only increase the polymerization degree ofthe exposed areas, i.e., would strengthen rather than deteriorate theimage. In addition, the present inventors have discovered that byheating the precursor in a pre-heating unit within a time period of lessthan 10 minutes after image-wise exposing, the stability of the latentimage is improved resulting in reproducing images of high resolution.

A further preferred embodiment of the present invention provides amethod for making a lithographic printing plate from a photopolymerplate precursor, which is perceived by the user as a method which doesnot require a processing step and wherein the exposed plate can be keptin ambient light for an unlimited time before being mounted on thepress, and wherein a visible image is provided before mounting the plateon the press. This can be achieved by adding a colorant to the coatingof the photopolymer plate. Since the non-printing areas of the coatingare removed in the gumming units, there is no risk of contamination ofthe fountain solution or ink during the start of the print job.

Other features, elements, steps, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with a preferred embodiment of the present invention, amethod includes the steps of: a) providing a lithographic printing plateprecursor including (i) a support having a hydrophilic surface or whichis provided with a hydrophilic layer, (ii) a coating on the supportincluding a photopolymerizable layer and, optionally, an intermediatelayer between the photopolymerizable layer and the support, wherein thephotopolymerizable layer includes a polymerizable compound, apolymerization initiator, and a binder, b) image-wise exposing thecoating in a plate setter, c) heating the precursor in a pre-heatingunit within a time period of less than 10 minutes after step (b), d)treating the precursor in a gumming station, including at least onegumming unit, whereby a gum solution is applied to the precursor,thereby removing non-exposed areas of the photopolymerizable layer fromthe support and gumming the plate in a single step.

In a preferred embodiment of the present invention, the printing plateprecursor is image-wise exposed off-press by a plate setter, i.e., alaser exposure apparatus suitable for image-wise exposing a precursor.The precursor used in a preferred embodiment is negative-working wherebyat the exposed areas the coating is hardened. Here, “hardened” meansthat the coating becomes insoluble or non-dispersible for the gumsolution and may be achieved through polymerization and/or crosslinkingof the coating.

After imaging, the plate precursor is heated, hereinafter also referredto as “pre-heat” or “pre-heat”, to enhance or to speed-up thepolymerization and/or crosslinking reaction. This pre-heat step iscarried out within a time period of less than 10 minutes, preferablyless than 5 minutes, more preferably less than 1 minute, most preferablythe pre-heat is carried out immediately after the image-wise exposing,i.e., within less than 30 seconds. There is no time limit before theheating may start, but the precursor is heated as soon as possible afterexposing, usually after a few seconds to transport the plate to thepre-heating unit and start the heating process. In this heating step,the precursor is heated at a temperature of preferably 80° C. to 150° C.and for a dwell time of preferably 5 seconds to 1 minute. Thepre-heating unit is preferably provided with heating elements such asIR-lamps, UV-lamps, heated air, a heated metal roll, etc.

Subsequently to the pre-heat step, the plate precursor is treated, i.e.,developed and gummed, in a gumming station which includes at least onegumming unit, preferably two gumming units, namely a first and a secondgumming unit. In the gumming unit(s), a gum solution is applied to thecoating of the precursor whereby the non-exposed areas of thephotopolymerizable layer is removed from the support and whereby thehydrophilic surface of the support at the non-exposed areas areprotected by adsorption of gum in a single step. When the precursor istreated with a gum solution firstly in a first gumming unit andsubsequently in a second gumming unit, an additional benefit is obtainedwhereby contamination of the gum solution of the second gumming unit isreduced, whereby an increase of the viscosity of the gum solution of thesecond gumming unit can be reduced or inhibited and that sludgeformation in the second gumming unit is retarded or inhibited. Thisresults in an improved lifetime of the gum solution of the secondgumming unit whereby this gum solution, optionally combined withregeneration (e.g., filtering) and/or adding replenishing solution, canbe used for developing and gumming a higher number of plates which savescosts and is friendly for the environment.

The development with a gum solution has the additional benefit that, dueto the remaining gum on the plate at the non-exposed areas, anadditional gumming step is not required to protect the surface of thesupport at the non-exposed areas. As a result, the precursor isprocessed and gummed in one single step and the obtained lithographicimage on the plate will not be affected by ambient daylight or bycontamination.

In the printing step, the plate is mounted on the plate cylinder of theprinting press and the printing process is started.

The Gum Solution

A gum solution is typically an aqueous liquid which includes one or moresurface protective compounds that are capable of protecting thelithographic image of a printing plate against contamination, e.g., byoxidation, fingerprints, fats, oils or dust, or damaging, e.g., byscratches during handling of the plate. Suitable examples of suchcompounds are film-forming hydrophilic polymers or surfactants. Thelayer that remains on the plate after treatment with the gum solutionpreferably includes between 0.005 and 20 g/m² of the surface protectivecompound, more preferably between 0.010 and 10 g/m², most preferablybetween 0.020 and 5 g/m².

In the present description, all concentrations of compounds present inthe gum solution are expressed as percentage by weight (wt. % or % w/w)relative to the ready-to-use gum solution, unless otherwise indicated. Agum solution may be normally supplied as a concentrated solution whichis diluted by the end user with water to a ready-to-use gum solutionbefore use according to the instructions of the supplier, usually 1 partof the gum is diluted with 1 part to 10 parts of water.

Preferred polymers for use as the protective compound in the gumsolution are gum arabic, pullulan, cellulose derivatives such ascarboxymethylcellulose, carboxyethylcellulose or methylcellulose,(cyclo)dextrin, poly(vinyl alcohol), poly(vinyl pyrrolidone),polysaccharide, homo- and copolymers of acrylic acid, methacrylic acidor acrylamide, a copolymer of vinyl methyl ether and maleic anhydride, acopolymer of vinyl acetate and maleic anhydride, or a copolymer ofstyrene and maleic anhydride. Highly preferred polymers are home- orcopolymers of monomers containing carboxylic, sulfonic, or phosphonicgroups or the salts thereof, e.g., (meth)acrylic acid, vinyl acetate,styrene sulfonic acid, vinyl sulfonic acid, vinyl phosphonic acid, oracrylamidopropane sulfonic acid.

Examples of surfactants for use as the surface protective agent includeanionic or nonionic surfactants. The gum solution may also include oneor more of the above hydrophilic polymers as the surface protectiveagent and, in addition, one or more surfactants to improve the surfaceproperties of the coated layer. The surface tension of the gum solutionis preferably from 20 to 50 mN/m.

The gum solution preferably includes an anionic surfactant, morepreferably an anionic surfactant wherein the anionic group is asulphonic acid group.

Examples of the anionic surfactant include aliphates, abietates,hydroxyalkanesulfonates, alkanesulfonates, dialkylsulfosuccinates,straight-chain alkylbenzenesulfonates, branched alkylbenzenesulfonates,alkylnaphthalenesulfonates, alkylphenoxypolyoxyethylenepropylsulfonates,salts of polyoxyethylene alkylsulfophenyl ethers, sodiumN-methyl-N-oleyltaurates, monoamide disodium N-alkylsulfosuccinates,petroleum sulfonates, sulfated castor oil, sulfated tallow oil, salts ofsulfuric esters of aliphatic alkylesters, salts of alkylsulfuric esters,sulfuric esters of polyoxyethylenealkylethers, salts of sulfuric estersof aliphatic monoglycerides, salts of sulfuric esters ofpolyoxyethylenealkylphenylethers, salts of sulfuric esters ofpolyoxyethylenestyrylphenylethers, salts of alkylphosphoric esters,salts of phosphoric esters of polyoxyethylenealkylethers, salts ofphosphoric esters of polyoxyethylenealkylphenylethers, partiallysaponified compounds of styrenemaleic anhydride copolymers, partiallysaponified compounds of olefin-maleic anhydride copolymers, andnaphthalenesulfonateformalin condensates. Particularly preferred amongthese anionic surfactants are dialkylsulfosuccinates, salts ofalkylsulfuric esters and alkylnaphthalenesulfonates.

Specific examples of suitable anionic surfactants include sodiumdodecylphenoxybenzene disulfonate, the sodium salt of alkylatednaphthalenesulfonate, disodium methylene-dinaphtalene-disulfonate,sodium dodecyl-benzenesulfonate, sulfonated alkyl-diphenyloxide,ammonium or potassium perfluoroalkylsulfonate, and sodiumdioctyl-sulfosuccinate.

Suitable examples of the nonionic surfactants include polyoxyethylenealkyl ethers, polyoxyethylene alkyl aryl ethers wherein the aryl groupmay be a phenyl group, a naphthyl group or an aromatic heterocyclicgroup, polyoxyethylene polystyryl phenyl ethers, polyoxyethylenepolyoxypropylene alkyl ethers, polyoxyethylene polyoxypropylene blockpolymers, partial esters of glycerinaliphatic acids, partial esters ofsorbitanaliphatic acid, partial esters of pentaerythritolaliphatic acid,propyleneglycolmonoaliphatic esters, partial esters of sucrosealiphaticacids, partial esters of polyoxyethylenesorbitanaliphatic acid, partialesters of polyoxyethylenesorbitolaliphatic acids,polyethyleneglycolaliphatic esters, partial esters ofpolyglycerinaliphatic acids, polyoxyethylenated castor oils, partialesters of polyoxyethyleneglycerinaliphatic acids, aliphaticdiethanolamides, N,N-bis-2-hydroxyalkylamines, polyoxyethylenealkylamines, triethanolaminealiphatic esters, and trialkylamine oxides.Particularly preferred among these nonionic surfactants arepolyoxyethylene alkylphenyl ethers, polyoxyethylene alkylnaphthylethers, and poloxyethylene-polyoxypropylene block polymers. Further,fluorinic and siliconic anionic and nonionic surfactants may besimilarly used.

Two or more of the above surfactants may be used in combination. Forexample, a combination of two or more different anionic surfactants or acombination of an anionic surfactant and a nonionic surfactant may bepreferred. The amount of such a surfactant is not specifically limitedbut is preferably from 0.01 to 30 wt. %, more preferably from 0.05 to 20wt. %.

According to a preferred embodiment of the present invention the gumsolution has a pH-value preferably between 3 and 9, more preferablybetween 4.5 and 8.5, most preferably between 5 and 7. The pH of the gumsolution is usually adjusted with a mineral acid, an organic acid, or aninorganic salt in an amount of from 0.01 to 15 wt. %, preferably from0.02 to 10 wt. %. Examples of the mineral acids include nitric acid,sulfuric acid, phosphoric acid, and metaphosphoric acid. Especiallyorganic acids are used as pH control agents and as desensitizing agents.Examples of the organic acids include carboxylic acids, sulfonic acids,phosphonic acids or salts thereof, e.g., succinates, phosphates,phosphonates, sulfates, and sulfonates. Specific examples of the organicacid include citric acid, acetic acid, oxalic acid, malonic acid,p-toluenesulfonic acid, tartaric acid, malic acid, lactic acid,levulinic acid, phytic acid, and organic phosphonic acid.

The gum solution further preferably includes an inorganic salt. Examplesof the inorganic salt include magnesium nitrate, monobasic sodiumphosphate, dibasic sodium phosphate, nickel sulfate, sodiumhexametaphosphate, and sodium tripolyphosphate. An alkali-metaldihydrogen phosphate such as KH₂PO₄ or NaH₂PO₄ is most preferred. Otherinorganic salts can be used as corrosion inhibiting agents, e.g.,magnesium sulfate or zinc nitrate. The mineral acid, organic acid, orinorganic salt may be used singly or in combination with one or morethereof.

In accordance with another preferred embodiment of the presentinvention, the gum solution as the developer in the processing of theplate preferably includes a mixture of an anionic surfactant and aninorganic salt. In this mixture the anionic surfactant is preferably ananionic surfactant with a sulphonic acid group, more preferably analkali-metal salt of a mono- or di-alkyl substituteddiphenylether-sulphonic acid, and the inorganic salt is preferably amono or dibasic phosphate salt, more preferably an alkali-metaldihydrogen phosphate, most preferably KH₂PO₄ or NaH₂PO₄.

In accordance with another preferred embodiment of the presentinvention, the gum solution including a mixture of an anionic surfactantand an inorganic salt preferably has a pH-value between 3 and 9, morepreferably between 4 and 8, most preferably between 5 and 7.

Besides the foregoing components, a wetting agent such as ethyleneglycol, propylene glycol, triethylene glycol, butylene glycol, hexyleneglycol, diethylene glycol, dipropylene glycol, glycerin, trimethylolpropan, and diglycerin may also be present in the gum solution. Thewetting agent may be used singly or in combination with one or morethereof. In general, the foregoing wetting agent is preferably used inan amount of from 1 to 25 wt. %.

Further, a chelate compound may be present in the gum solution. Calciumion and other impurities contained in the diluting water can haveadverse effects on printing and thus cause the contamination of printedmatter. This problem can be eliminated by adding a chelate compound tothe diluting water. Preferred examples of such a chelate compoundinclude organic phosphonic acids or phosphonoalkanetricarboxylic acids.Specific examples are potassium or sodium salts ofethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid,triethylenetetraminehexaacetic acid,hydroxyethylethylenediaminetriacetic acid, nitrilotriacetic acid,1-hydroxyethane-1,1-diphosphonic acid and aminotri(methylenephosphonicacid). Besides these sodium or potassium salts of these chelatingagents, organic amine salts are useful. The preferred amount of such achelating agent to be added is from 0.001 to 5 wt. % relative to the gumsolution in diluted form.

Further, an antiseptic and an anti-foaming agent may be present in thegum solution. Examples of such an antiseptic include phenol, derivativesthereof, formalin, imidazole derivatives, sodium dehydroacetate,4-isothiazoline-3-one derivatives, benzoisothiazoline-3-one,benztriazole derivatives, amidineguanidine derivatives, quaternaryammonium salts, pyridine derivatives, quinoline derivatives, guanidinederivatives, diazine, triazole derivatives, oxazole and oxazinederivatives. The preferred amount of such an antiseptic to be added issuch that it can exert a stable effect on bacteria, fungi, yeast or thelike. Though depending on the kind of bacteria, fungi and yeast, it ispreferably from 0.01 to 4 wt. % relative to the gum solution in dilutedform. Further, preferably, two or more antiseptics may be used incombination to exert an aseptic effect on various fungi and bacteria.The anti-foaming agent is preferably silicone anti-foaming agents. Amongthese anti-foaming agents, either an emulsion dispersion type orsolubilized type anti-foaming agent may be used. The proper amount ofsuch an anti-foaming agent to be added is from 0.001 to 1.0 wt. %relative to the gum solution in diluted form.

Besides the foregoing components, an ink receptivity agent may bepresent in the gum solution if desired. Examples of such an inkreceptivity agent include turpentine oil, xylene, toluene, low heptane,solvent naphtha, kerosene, mineral spirit, hydrocarbons such aspetroleum fraction having a boiling point of about 120° C. to about 250°C., diester phthalates (e.g., dibutyl phthalate, diheptyl phthalate,di-n-octyl phthalate, di(2-ethylhexyl) phthalate, dinonyl phthalate,didecyl phthalate, dilauryl phthalate, butylbenzyl phthalate), aliphaticdibasic esters (e.g., dioctyl adipate, butylglycol adipate, dioctylazelate, dibutyl sebacate, di(2-ethylhexyl) sebacate dioctyl sebacate),epoxidated triglycerides (e.g., epoxy soyabean oil), ester phosphates(e.g., tricresyl phosphate, trioctyl phosphate, trischloroethylphosphate) and plasticizers having a solidification point of 15° C. orless and a boiling point of 300° C. or more at one atmospheric pressuresuch as esters of benzoates (e.g., benzyl benzoate). Examples of othersolvents which can be used in combination with these solvents includeketones (e.g., cyclohexanone), halogenated hydrocarbons (e.g., ethylenedichloride), ethylene glycol ethers (e.g., ethylene glycol monomethylether, ethylene glycol monophenyl ether, ethylene glycol monobutylether), aliphatic acids (e.g., caproic acid, enathic acid, caprylicacid, pelargonic acid, capric acid, undecylic acid, lauric acid,tridecylic acid, myristic acid, pentadecylic acid, palmitic acid,heptadecylic acid, stearic acid, nonadecanic acid, arachic acid, behenicacid, lignoceric acid, cerotic acid, heptacosanoic acid, montanic acid,melissic acid, lacceric acid, isovaleric acid) and unsaturated aliphaticacids (e.g., acrylic acid, crotonic acid, isocrotonic acid, undecyclicacid, oleic acid, elaidic acid, cetoleic acid, erucic acid, butecidicacid, sorbic acid, linoleic acid, linolenic acid, arachidonic acid,propiolic acid, stearolic acid, clupanodonic acid, tariric acid, licanicacid). Preferably, it is an aliphatic acid which is liquid at atemperature of 50° C., more preferably has from 5 to 25 carbon atoms,most preferably has from 8 to 21 carbon atoms. The ink receptivity agentmay be used singly or in combination with one or more thereof. The inkreceptivity agent is preferably used in an amount of from 0.01 to 10 wt.%, more preferably from 0.05 to 5 wt. %. The foregoing ink receptivityagent may be present as an oil-in-water emulsion or may be solubilizedwith the aid of a solubilizing agent.

The viscosity of the gum solution can be adjusted to a value of, e.g.,between 1.7 and 5 mPa·s, by adding viscosity increasing compounds, suchas poly(ethylene oxide) or polyvinylalcohol, e.g., having a molecularweight between 10⁴ and 10⁷. Such compounds can be present in aconcentration of 0.01 to 10 g/l.

A baking gum has a similar composition as described above, with theadditional preference towards compounds that do not evaporate at theusual bake temperatures. Baking gum solutions or baking gummingsolutions can be aqueous solutions of sodium dodecyl phenoxy benzenedisulphonate, alkylated naphthalene sulphonic acid, sulphonated alkyldiphenyl oxide, methylene dinaphtalene sulphonic acid, etc. Othergumming solutions contain a hydrophilic polymer component and an organicacid component. Still other baking gumming solutions contain thepotassium salt of the hydroxyethylidene diphosphonic acid. Still otherbaking gumming solutions contain a sulphosuccinamate compound andphosphoric acid.

The contact angle between the baking gum solution and the plate ispreferably lowered by adding at least one surfactant. Preferredsurfactants are non-ionic polyglycols and perfluorated aliphaticpolyester acrylates.

In another preferred embodiment, the baking gumming solutions include(a) water, (b) at least one hydrophilic polymer, and (c) at least onecomponent selected from water soluble organic acids including at leasttwo acid functions and being selected from benzene carboxylic acid, abenzene sulphonic acid, a benzene phosphonic acid, an alkane phosphonicacid and water soluble salts thereof. The mentioned compounds (b) and(c) which are dissolved in the aqueous solution in accordance with apreferred embodiment of the present invention are such that they do notevaporate at the customary baking temperatures. The protective layerwhich is formed remains water-soluble, even after baking, and can bereadily removed without damaging the printing plate.

Component (b) includes, in particular, the following hydrophilicpolymers: N-polyvinyl-pyrrolidone, polyvinylmethylether, copolymerscontaining ethylene units and maleic anhydride units, homopolymers orcopolymers containing vinyl phosphonic acid units, vinyl methylphosphinic acid units and/or acrylic acid units and/or a polyalkyleneglycol, such as polyethylene glycol.

Component (c) includes in particular: benzene disulphonic acids, benzenepolycarboxylic acids having from 3 to 6 carboxyl groups, alkanediphosphonic acids which having from 1 to 3 carbon atoms in the alkanegroup, carboxyl group containing alkane diphosphonic acids which havefrom 5 to 9 carbon atoms in the alkane group, and/or one of thewater-soluble salts of these acids (preferably alkali metal salts orammonium salts). Specific examples of component (c) includebenzene-1,3-disulphonic acid, benzene-1,2,4-tricarboxylic acid(trimellitic acid), benzene 1,2,4,5-tetracarboxylic acid (pyromelliticacid), benzene hexacarboxylic acid (mellitic acid), methane diphosphonicacid (diphosphono methane), 4,4-diphosphono-heptane-1,7-dioic acid(3,3-diphosphone-pimeic acid), and the sodium salts of these acids. Inother preferred embodiments, the baking gumming solution canadditionally contain hydroxy-polycarboxylic acids, such as citric acidand/or the salts thereof, water soluble alkanediols having at least 4carbon atoms, such as hexanediol-(1,6) and surfactants (preferablyanionic or non-ionic surfactants) such as alkyl aryl sulphonates, alkylphenol ether sulphonates and a natural surfactant (e.g., Saponin).Specific examples of suitable baking gum solutions, ingredients andconcentrations thereof, can be found in e.g., EP-A 222 297, EP-A 1 025992, DE-A 2 626 473, and U.S. Pat. No. 4,786,581.

The Support

A particularly preferred lithographic support is an electrochemicallygrained and anodized aluminum support. Graining and anodizing ofaluminum supports is well known. The acid used for graining can be,e.g., nitric acid or sulfuric acid. The acid used for grainingpreferably includes hydrogen chloride. Also mixtures of, e.g., hydrogenchloride and acetic acid can be used. The relationship betweenelectrochemical graining and anodizing parameters such as electrodevoltage, nature, and concentration of the acid electrolyte or powerconsumption on the one hand and the obtained lithographic quality interms of Ra and anodic weight (g/m² of Al₂O₃ formed on the aluminumsurface) on the other hand is well known. More details about therelationship between various production parameters and Ra or anodicweight can be found in, e.g., the article “Management of Change in theAluminium Printing Industry” by F. R. Mayers, published in the ATBMetallurgie Journal, Vol. 42 No. 1-2, (2002), page 69.

The anodized aluminum support may be subject to a so-called post-anodictreatment to improve the hydrophilic properties of its surface. Forexample, the aluminum support may be silicated by treating its surfacewith a sodium silicate solution at elevated temperature, e.g., 95° C.Alternatively, a phosphate treatment may be applied which involvestreating the aluminum oxide surface with a phosphate solution that mayfurther contain an inorganic fluoride. Further, the aluminum oxidesurface may be rinsed with a citric acid or citrate solution. Thistreatment may be carried out at room temperature or may be carried outat a slightly elevated temperature of about 30 to 50° C. A furtherinteresting treatment involves rinsing the aluminum oxide surface with abicarbonate solution. Still further, the aluminum oxide surface may betreated with polyvinylphosphonic acid, polyvinylmethylphosphonic acid,phosphoric acid esters of polyvinyl alcohol, polyvinylsulfonic acid,polyvinylbenzenesulfonic acid, sulfuric acid esters of polyvinylalcohol, and acetals of polyvinyl alcohols formed by reaction with asulfonated aliphatic aldehyde.

Another useful post-anodic treatment may be carried out with a solutionof polyacrylic acid or a polymer including at least 30 mol % of acrylicacid monomeric units, e.g., GLASCOL E15, a polyacrylic acid,commercially available from ALLIED COLLOIDS.

The grained and anodized aluminum support may be a sheet-like materialsuch as a plate or it may be a cylindrical element such as a sleevewhich can be slid around a print cylinder of a printing press.

The support can also be a flexible support, which may be provided with ahydrophilic layer, hereinafter called ‘base layer’. The flexible supportis, e.g., paper, plastic film or aluminum. Preferred examples of plasticfilm are polyethylene terephthalate film, polyethylene naphthalate film,cellulose acetate film, polystyrene film, polycarbonate film, etc. Theplastic film support may be opaque or transparent.

The base layer is preferably a cross-linked hydrophilic layer obtainedfrom a hydrophilic binder cross-linked with a hardening agent such asformaldehyde, glyoxal, polyisocyanate, or a hydrolyzedtetra-alkylorthosilicate. The latter is particularly preferred. Thethickness of the hydrophilic base layer may vary in the range of 0.2 to25 μm and is preferably 1 to 10 μm. More details of preferredembodiments of the base layer can be found in, e.g., EP-A 1 025 992.

The Coating

The coating on the support includes at least one layer including aphotopolymerizable composition, the layer hereinafter also referred toas “photopolymerizable layer”. The coating may further include on thephotopolymerizable layer, an oxygen-barrier layer which includes awater-soluble or water-swellable polymer, the barrier layer hereinafteralso referred to as “top layer” or “overcoat” or “overcoat layer”. Thecoating may further include an intermediate layer between thephotopolymerizable layer and the support.

The thickness of the coating preferably ranges between 0.4 and 10 g/m²,more preferably between 0.5 and 5 g/m², most preferably between 0.6 and3 g/m².

The photopolymerizable layer includes a polymerizable compound, apolymerization initiator capable of hardening the polymerizable compoundin the exposed areas, and a binder. The photopolymerizable layer mayfurther include an adhesion promoting compound.

The photopolymerizable layer has a coating thickness preferably rangingbetween 0.4 and 5.0 g/m², more preferably between 0.5 and 3.0 g/m², mostpreferably between 0.6 and 2.2 g/m².

The Adhesion Promoting Compound

The adhesion promoting compound is a compound capable of interactingwith the support, preferably a compound having an addition-polymerizableethylenically unsaturated bond and a functional group capable ofinteracting with the support, more preferably a functional group capableof interacting with a grained and anodized aluminum support. By“interacting” it is understood each type of physical and/or chemicalreaction or process whereby, between the functional group and thesupport, a bond is formed which can be a covalent bond, an ionic bond, acomplex bond, a coordinate bond or a hydrogen-bridge bond, and which canbe formed by an adsorption process, a chemical reaction, an acid-basereaction, a complex-forming reaction, or a reaction of a chelating groupor a ligand. The adhesion promoting compound may be present in thephotopolymerizable layer and/or in an intermediate layer between thephotopolymerizable layer and the support.

The adhesion promoting compound may be selected from at least one of thelow molecular weight compounds or polymeric compounds as described inEP-A 851 299 from lines 22 on page 3 to line 1 on page 4, EP-A 1 500 498from paragraph [0023] on page 7 to paragraph [0052] on page 20, EP-A 1495 866 paragraph [0030] on page 5 to paragraph [0049] on page 11, EP-A1 091 251 from paragraph [0014] on page 3 to paragraph [0018] on page20, and EP-A 1 520 694 from paragraph [0023] on page 6 to paragraph[0060] on page 19. Preferred compounds are those compounds which includea phosphate or phosphonate group as a functional group capable ofadsorbing on the aluminum support and which include anaddition-polymerizable ethylenic double bond reactive group, especiallythose described in EP-A 851 299 from lines 22 on page 3 to line 1 onpage 4 and EP-A 1 500 498 from paragraph [0023] on page 7 to paragraph[0052] on page 20. Also preferred are those compounds which includetri-alkyl-oxy silane groups, hereinafter also referred to as “trialkoxysilane” groups, wherein the alkyl is preferably methyl or ethyl, orwherein the trialkyloxy silane groups are at least partially hydrolyzedto silanol groups, as a functional group capable of adsorbing on thesupport, especially silane coupling agents having anaddition-polymerizable ethylenic double bond reactive group as describedin EP-A 1 557 262 paragraph [0279] on page 49 and EP-A 1 495 866paragraph [0030] on page 5 to paragraph [0049] on page 11.

The adhesion promoting compound may be present in the photopolymerizablelayer in an amount ranging between 1 and 50 wt %, preferably between 3and 30 wt %, more preferably between 5 and 20 wt % of the non-volatilecomponents of the composition.

The adhesion promoting compound may be present in the intermediate layerin an amount of at least 50 wt %, preferably at least 80 wt %, morepreferably at least 90 wt %, most preferably 100 wt % of thenon-volatile components of the composition.

The optional intermediate layer has a coating thickness preferablyranging between 0.001 and 1.5 g/m², more preferably between 0.003 and1.0 g/m², most preferably between 0.005 and 0.7 g/m².

The Polymerizable Compound and the Polymerization Initiator

According to one preferred embodiment of the present invention, thepolymerizable monomer or oligomer is a monomer or oligomer including atleast one epoxy or vinyl ether functional group and the initiator is aBronsted acid generator capable of generating a free acid, optionally inthe presence of a sensitizer, upon exposure, hereinafter the initiatoralso referred to as “cationic photoinitiator” or “cationic initiator”.

Suitable polyfunctional epoxy monomers include, for example,3,4-epoxycyclohexylmethyl-3,4-epoxycyclohex-ane carboxylate,bis-(3,4-epoxycyclohexymethyl) adipate, difunctional bisphenolAepichlorohydrin epoxy resin, and multifunctionalepichlorohydrinitetraphenylol ethane epoxy resin.

Suitable cationic photoinitiators include, for example, triarylsulfoniumhexafluoroantimonate, triarylsulfonium hexafluorophosphate,diaryliodonium hexafluoroantimonate, and haloalkyl substituteds-triazine. It is noted that most cationic initiators are also freeradical initiators because, in addition to generating Bronsted acid,they also generate free radicals during photo or thermal decomposition.

According to a more preferred embodiment of the present invention, thepolymerizable monomer or oligomer is a ethylenically unsaturatedcompound, having at least one terminal ethylenic group, hereinafter alsoreferred to as “free-radical polymerizable monomer”, and the initiatoris a compound, capable of generating a free radical, optionally in thepresence of a sensitizer, upon exposure, hereinafter the initiator alsoreferred to as “free radical initiator”.

Suitable free-radical polymerizable monomers include, for example,multifunctional (meth)acrylate monomers (such as (meth)acrylate estersof ethylene glycol, trimethylolpropane, pentaerythritol, ethoxylatedethylene glycol and ethoxylated trimethylolpropane, multifunctionalurethanated (meth)acrylate, and epoxylated (meth)acrylate), andoligomeric amine diacrylates. The (meth)acrylic monomers may also haveother double bond or epoxide group, in addition to (meth)acrylate group.The (meth)acrylate monomers may also contain an acidic (such ascarboxylic acid) or basic (such as amine) functionality.

Any free radical initiator capable of generating a free radical directlyor in the presence of a sensitizer upon exposure can be used as a freeradical initiator. Suitable free-radical initiators include, forexample, the derivatives of acetophenone (such as2,2-dimethoxy-2-phenylacetophenone, and 2-methyl-1-[4-(methylthio)phenyll-2-morpholino propan-1-one); benzophenone; benzil; ketocoumarin(such as 3-benzoyl-7-methoxy coumarin and 7-methoxy coumarin); xanthone;thioxanthone; benzoin or an alkyl-substituted anthraquinone; onium salts(such as diaryliodonium hexafluoroantimonate, diaryliodonium triflate,(4-(2-hydroxytetradecyl-oxy)-phenyl) phenyliodoniumhexafluoroantimonate, triarylsulfonium hexafluorophosphate,triarylsulfonium p-toluenesulfonate, (3-phenylpropan-2-onyl) triarylphosphonium hexafluoroantimonate, and N-ethoxy(2-methyl)pyridiniumhexafluorophosphate, and onium salts as described in U.S. Pat. Nos.5,955,238, 6,037,098, and 5,629,354); borate salts (such astetrabutylammonium triphenyl(n-butyl)borate, tetraethylammoniumtriphenyl(n-butyl)borate, diphenyliodonium tetraphenylborate, andtriphenylsulfonium triphenyl(n-butyl)borate, and borate salts asdescribed in U.S. Pat. Nos. 6,232,038 and 6,218,076,); haloalkylsubstituted s-triazines (such as2,4-bis(trichloromethyl)-6-(p-methoxy-styryl)-s-triazine,2,4-bis(trichloromethyl)-6-(4-methoxy-naphth-1-yl)-s-triazine,2,4-bis(trichloromethyl)-6-piperonyl-s-triazine, and2,4-bis(trichloromethyl)-6-[(4-ethoxy-ethylenoxy)-phen-1-yl]-s-triazine,and s-triazines as described in U.S. Pat. Nos. 5,955,238, 6,037,098,6,010,824 and 5,629,354); and titanocene(bis(etha.9-2,4-cyclopentadien-1-yl)bis[2,6-difluoro-3-(1H-pyrrol-1-yl)phenyl) titanium). Onium salts,borate salts, and s-triazines are preferred free radical initiators.Diaryliodonium salts and triarylsulfonium salts are preferred oniumsalts. Triarylalkylborate salts are preferred borate salts.Trichloromethyl substituted s-triazines are preferred s-triazines.

Known photopolymerization initiators can be used in the composition ofthe preferred embodiments of the present invention. In a preferredembodiment of the present invention, the photopolymerizable compositionincludes a hexaarylbisimidazole (HABI; dimer of triaryl-imidazole)compound as a photopolymerization initiator alone or in combination withfurther photoinitiators.

A procedure for the preparation of hexaarylbisimidazoles is described inDE 1470 154 and their use in photopolymerizable compositions isdocumented in EP 24 629, EP 107 792, U.S. Pat. No. 4,410,621, EP 215453, and DE 3 211 312. Preferred derivatives are, e.g.,2,4,5,2′,4′,5′-hexaphenylbisimidazole,2,2′-bis(2-chlorophenyl)-4,5,4′,5′-tetraphenylbisimidazole,2,2′-bis(2-bromophenyl)-4,5,4′,5′-tetraphenylbisimidazole,2,2′-bis(2,4-dichlorophenyl)-4,5,4′,5′-tetraphenylbisimidazole,2,2′-bis(2-chlorophenyl)-4,5,4′,5′-tetrakis(3-methoxyphenyl)bisimidazole,2,2′-bis(2-chlorophenyl)-4,5,4′,5′-tetrakis(3,4,5-trimethoxyphenyl)-bisimidazole,2,5,2′,5′-tetrakis(2-chlorophenyl)-4,4′-bis(3,4-dimethoxyphenyl)bisimidazole,2,2′-bis(2,6-dichlorophenyl)-4,5,4′,5′-tetraphenylbisimidazole,2,2′-bis(2-nitrophenyl)-4,5,4′,5′-tetraphenylbisimidazole,2,2′-di-o-tolyl-4,5,4′,5′-tetraphenylbisimidazole,2,2′-bis(2-ethoxyphenyl)-4,5,4′,5′-tetraphenylbisimidazole, and2,2′-bis(2,6-difluorophenyl)-4,5,4′,5′-tetraphenylbisimidazole. Theamount of the HABI photoinitiator typically ranges from 0.01 to 30% byweight, preferably from 0.5 to 20% by weight, relative to the totalweight of the non volatile components of the photopolymerizablecomposition.

A very high sensitivity can be obtained in the context of a preferredembodiment of the present invention by the combination of an opticalbrightener as the sensitizer and a hexaarylbisimidazole as thephotoinitiator.

Suitable classes of photoinitiators other than hexaarylbisimidazolecompounds include aromatic ketones, aromatic onium salts, organicperoxides, thio compounds, ketooxime ester compounds, borate compounds,azinium compounds, metallocene compounds, active ester compounds andcompounds having a carbon-halogen bond, but preferably the compositionincludes a non-boron including photopolymerization initiator andparticularly preferred the photopolymerization initiator includes noboron compound. Many specific examples of photoinitiators suitable forthe preferred embodiments of the present invention can be found in EP-A1 091 247. Other preferred initiators are trihalo methyl sulphones.

Preferably hexaarylbisimidazole compounds and/or metallocene compoundsare used alone or in combination with other suitable photoinitiators, inparticular with aromatic ketones, aromatic onium salts, organicperoxides, thio compounds, ketoxime ester compounds, azinium compounds,active ester compounds, or compounds having a carbon halogen bond.

In a preferred embodiment of the present invention, thehexaarylbisimidazole compounds make more than 50 mol-%, preferably atleast 80 mol-% and particularly preferred at least 90 mol-% of all thephotoinitiators used in the photopolymerizable composition.

According to another preferred embodiment of the present invention, thepolymerizable monomer or oligomer may be a combination of a monomer oroligomer including at least one epoxy or vinyl ether functional groupand a polymerizable ethylenically unsaturated compound, having at leastone terminal ethylenic group, and the initiator may be a combination ofa cationic initiator and a free-radical initiator. A monomer or oligomerincluding at least one epoxy or vinyl ether functional group and apolymerizable ethylenically unsaturated compound, having at least oneterminal ethylenic group, can be the same compound wherein the compoundcontains both ethylenic group and epoxy or vinyl ether group. Examplesof such compounds include epoxy functional acrylic monomers, such asglycidyl acrylate. The free radical initiator and the cationic initiatorcan be the same compound If the compound is capable of generating both afree radical and free acid. Examples of such compounds include variousonium salts such as diaryliodonium hexafluoroantimonate and s-triazinessuch as2,4-bis(trichloromethyl)-6-[(4-ethoxyethylenoxy)-phen-1-yl]-s-triazinewhich are capable of generating both a free radical and free acid in thepresence of a sensitizer.

The photopolymerizable layer may also include a multifunctional monomer.This monomer contains at least two functional groups selected from anethylenically unsaturated group and/or an epoxy or vinyl ether group.Particular multifunctional monomers for use in the photopolymer coatingare disclosed in U.S. Pat. No. 6,410,205, U.S. Pat. No. 5,049,479, EP1079276, EP 1369232, EP 1369231 EP 1341040, U.S. 2003/0124460, EP1241002, EP 1288720, and in the reference book including the citedreferences: Chemistry & Technology UV & EB Formulation for Coatings,Inks & Paints, Volume 2, Prepolymers and Reactive Diluents for UV and EBCurable Formulations by N. S. Allen, M. A. Johnson, P. K. T. Oldring, M.S. Salim, Edited by P. K. T. Oldring, 1991, ISBN 0 947798102.Particularly preferred are urethane (meth)acrylate multifunctionalmonomers, which can be used alone or in combination with other(meth)acrylate multifunctional monomers.

The photopolymerizable layer may also include a co-initiator. Typically,a co-initiator is used in combination with a free radical initiatorand/or cationic initiator. Particular co-initiators for use in thephotopolymer coating are disclosed in U.S. Pat. No. 6,410,205, U.S. Pat.No. 5,049,479, EP 1079276, 1369232, EP 1369231, EP 1341040, U.S.2003/0124460, EP 1241002, EP 1288720, and in the reference bookincluding the cited references: Chemistry & Technology UV & EBFormulation for Coatings, Inks & Paints, Volume 3, Photoinitiators forFree Radical and Cationic Polymerisation by K. K. Dietliker, Edited byP. K. T. Oldring, 1991, ISBN 0 947798161.

The photopolymerizable layer may also include an inhibitor. Particularinhibitors for use in the photopolymer coating are disclosed in U.S.Pat. No. 6,410,205, EP 1288720, and WO 2005/109103.

The Binder of the Photopolymerizable Layer

The photopolymerizable layer may also include a binder. The binder canbe selected from a wide series of organic polymers. Compositions ofdifferent binders can also be used. Useful binders include for examplechlorinated polyalkylene (in particular chlorinated polyethylene andchlorinated polypropylene), polymethacrylic acid alkyl esters or alkenylesters (in particular polymethyl (meth)acrylate, polyethyl(meth)acrylate, polybutyl (meth)acrylate, polyisobutyl (meth)acrylate,polyhexyl (meth)acrylate, poly(2-ethylhexyl) (meth)acrylate andpolyalkyl (meth)acrylate copolymers of (meth) acrylic acid alkyl estersor alkenyl esters with other copolymerizable monomers (in particularwith (met)acrylonitrile, vinyl chloride, vinylidene chloride, styreneand/or butadiene), polyvinyl chloride (PVC,vinylchloride/(meth)acrylonitrile copolymers, polyvinylidene chloride(PVDC), vinylidene chloride/(meth)acrylonitrile copolymers, polyvinylacetate, polyvinyl alcohol, polyvinyl pyrrolidone, copolymers of vinylpyrrolidone or alkylated vinyl pyrrolidone, polyvinyl caprolactam,copolymers of vinyl caprolactam, poly (meth)acrylonitrile,(meth)acrylonitrile/styrene copolymers, (meth)acrylamide/alkyl(meth)acrylate copolymers, (meth)acrylonitrile/butadiene/styrene (ABS)terpolymers, polystyrene, poly(α-methylstyrene), polyamides,polyurthanes, polyesters, methyl cellulose, ethylcellulose, acetylcellulose, hydroxy-(C₁-C₄-alkyl) cellulose, carboxymethyl cellulose,polyvinyl formal and polyvinyl butyral. Particularly preferred bindersare polymers having vinylcaprolactam, vinylpyrrolidone or alkylatedvinylpyrrolidone as monomeric units. Alkylated vinylpyrrolidone polymerscan be obtained by grafting alfa-olefines onto the vinylpyrrolidonepolymer backbone. Typical examples of such products are the Agrimer ALGraft polymers commercially available from ISP. The length of thealkylation group may vary from C₄ to C₃₀. Other useful binders arebinders containing carboxyl groups, in particular copolymers containingmonomeric units of α,β-unsaturated carboxylic acids or monomeric unitsof α,β-unsaturated dicarboxylic acids (preferably acrylic acid,methacrylic acid, crotonic acid, vinylacetic acid, maleic acid oritaconic acid). The term “copolymers” is to be understood in the contextof the preferred embodiments of the present invention as polymerscontaining units of at least 2 different monomers, thus also terpolymersand higher mixed polymers. Particular examples of useful copolymers arethose containing units of (meth)acrylic acid and units of alkyl(meth)acrylates, allyl (meth)acrylates and/or (meth)acrylonitrile aswell as copolymers containing units of crotonic acid and units of alkyl(meth)acrylates and/or (meth)acrylonitrile and vinylacetic acid/alkyl(meth)acrylate copolymers. Also suitable are copolymers containing unitsof maleic anhydride or maleic acid monoalkyl esters. Among these are,for example, copolymers containing units of maleic anhydride andstyrene, unsaturated ethers or esters or unsaturated aliphatichydrocarbons and the esterification products obtained from suchcopolymers. Further suitable binders are products obtainable from theconversion of hydroxyl-containing polymers with intramoleculardicarboxylic anhydrides. Further useful binders are polymers in whichgroups with acid hydrogen atoms are present, some or all of which areconverted with activated isocyanates. Examples of these polymers areproducts obtained by conversion of hydroxyl-containing polymers withaliphatic or aromatic sulfonyl isocyanates or phosphinic acidisocyanates. Also suitable are polymers with aliphatic or aromatichydroxyl groups, for example copolymers containing units of hydroxyalkyl(meth)acrylates, allyl alcohol, hydroxystyrene or vinyl alcohol, as wellas epoxy resins, provided they carry a sufficient number of free OHgroups. Particular useful binder and particular useful reactive bindersare disclosed in EP 1 369 232, EP 1 369 231, EP 1 341 040, U.S.2003/0124460, EP 1 241 002, EP 1 288 720, U.S. Pat. No. 6,027,857, U.S.Pat. No. 6,171,735, and U.S. Pat. No. 6,420,089.

The organic polymers used as binders have a typical mean molecularweight M_(w) between 600 and 700,000, preferably between 1,000 and350,000. Preference is further given to polymers having an acid numberbetween 10 to 250, preferably 20 to 200, or a hydroxyl number between 50and 750, preferably between 100 and 500. The amount of binder(s)generally ranges from 10 to 90% by weight, preferably 20 to 80% byweight, relative to the total weight of the non-volatile components ofthe composition.

Also, particularly suitable binders are copolymers of vinylacetate andvinylalcohol, preferably including vinylalcohol in an amount of 10 to 98mol % vinylalcohol, more preferably between 35 and 95 mol %, mostpreferably 40 and 75 mol %, best results are obtained with 50 to 65 mol% vinylalcohol. The ester-value, measured by the method as defined inDIN 53 401, of the copolymers of vinylacetate and vinylalcohol rangespreferably between 25 and 700 mg KOH/g, more preferably between 50 and500 mg KOH/g, most preferably between 100 and 300 mg KOH/g. Theviscosity of the copolymers of vinylacetate and vinylalcohol aremeasured on a 4 weight % aqueous solution at 20° C. as defined in DIN 53015 and the viscosity ranges preferably between 3 and 60 mPa·s, morepreferably between 4 and 30 mPa·s, most preferably between 5 and 25mPa·s. The average molecular weight M_(w) of the copolymers ofvinylacetate and vinylalcohol ranges preferably between 5,000 and500,000 g/mol, more preferably between 10,000 and 400,000 g/mol, mostpreferably between 15,000 and 250,000 g/mol. Other preferred binders aredisclosed in EP 152 819 B1 on page 2 lines 50-page 4 line 20, and in EP1 043 627 B1 on paragraph [0013] on page 3.

In another preferred embodiment, the polymeric binder includes ahydrophobic backbone, and pendant groups including for example ahydrophilic poly(alkylene oxide) segment. The polymeric binder may alsoinclude pendant cyano groups attached to the hydrophobic backbone. Acombination of such binders may also be employed. Generally thepolymeric binder is a solid at room temperature, and is typically anon-elastomeric thermoplastic. The polymeric binder includes bothhydrophilic and hydrophobic regions, which is thought to be importantfor enhancing differentiation of the exposed and unexposed areas byfacilitating developability. Generally the polymeric binder ischaracterized by a number average molecular weight (Mn) in the rangefrom about 10,000 to 250,000, more commonly in the range from about25,000 to 200,000. The polymerizable composition may include discreteparticles of the polymeric binder. Preferably, the discrete particlesare particles of the polymeric binder which are suspended in thepolymerizable composition. The presence of discrete particles tends topromote developability of the unexposed areas. Specific examples of thepolymeric binders according to this preferred embodiment are describedin U.S. Pat. No. 6,899,994, U.S. 2004/0260050, U.S. 2005/0003285, U.S.2005/0170286, and U.S. 2005/0123853. In addition to the polymericbinder, the imageable layer may optionally include one or moreco-binders. Typical co-binders are water-soluble or water-dispersiblepolymers, such as, cellulose derivatives, poly vinyl alcohol, polyacrylic acid poly(meth)acrylic acid, poly vinyl pyrrolidone,polylactide, poly vinyl phosphonic acid, synthetic co-polymers, such asthe co-polymer of an alkoxy polyethylene glycol (meth)acrylate. Specificexamples of co-binders are described in U.S. 2004/0260050, U.S.2005/0003285, and U.S. 2005/0123853. Printing plate precursors, theimageable layer of which includes a binder and optionally a co-binderaccording to a preferred embodiment, and described in more detail inU.S. 2004/0260050, U.S. 2005/0003285, and U.S. 2005/0123853, optionallyinclude a top coat and an interlayer.

Surfactant

Various surfactants may be added into the photopolymerizable layer toallow or enhance the developability of the precursor with a gumsolution. Both polymeric and small molecule surfactants can be used.Nonionic surfactants are preferred. Preferred nonionic surfactants arepolymers and oligomers containing one or more polyether (such aspolyethylene glycol, polypropylene glycol, and copolymer of ethyleneglycol and propylene glycol) segments. Examples of preferred nonionicsurfactants are block copolymers of propylene glycol and ethylene glycol(also called block copolymer of propylene oxide and ethylene oxide);ethoxylated or propoxylated acrylate oligomers; and polyethoxylatedalkylphenols and polyethoxylated fatty alcohols. The nonionic surfactantis preferably added in an amount ranging between 0.1 and 30% by weightof the coating, more preferably between 0.5 and 20%, and most preferablybetween 1 and 15%.

Sensitizer

The photocurable composition may also include a sensitizer. Highlypreferred sensitizers are violet light absorbing sensitizers, having anabsorption spectrum between 350 nm and 450 nm, preferably between 370 nmand 420 nm, more preferably between 390 nm and 415 nm. Particularlypreferred sensitizers are disclosed in EP 1 349 006 paragraphs [0007] to[0009], WO 2005/029187, and WO2004/047930, including the citedreferences in these patent applications. Other highly preferredsensitizers are infrared light absorbing dyes, having an absorptionspectrum between 750 nm and 1300 nm, preferably between 780 nm and 1200nm, more preferably between 800 nm and 1100 nm. Particular preferredsensitizers are heptamethinecyane dyes, especially the dyes disclosed inEP 1 359 008 paragraphs [0030] to [0032]. Other preferred sensitizersare blue, green, or red light absorbing sensitizers, having anabsorption spectrum between 450 nm and 750 nm. Useful sensitizers can beselected from the sensitizing dyes disclosed in U.S. Pat. No. 6,410,205,U.S. Pat. No. 5,049,479, EP 1 079 276, EP 1 369 232, EP 1 369 231, EP 1341 040, U.S. 2003/0124460, EP 1 241 002, and EP 1 288 720.

Colorant

The photopolymerizable layer or another layer of the coating may alsoinclude a colorant. The colorant can be present in thephotopolymerizable layer or in a separate layer below or above thephotopolymerizable layer. After processing with a gum solution, at leasta portion of the colorant remains on the hardened coating areas, and avisible image can be produced on the support by removing the coating,including the colorant, at the non-exposed areas in the gum processing.

The colorant can be a dye or a pigment. A dye or pigment can be used asa colorant when the layer, including the dye or pigment, is colored forthe human eye.

The colorant can be a pigment. Various types of pigments can be usedsuch as organic pigments, inorganic pigments, carbon black, metallicpowder pigments, and fluorescent pigments. Organic pigments arepreferred.

Specific examples of organic pigments include quinacridone pigments,quinacridonequinone pigments, dioxazine pigments, phthalocyaninepigments, anthrapyrimidine pigments, anthanthrone pigments, indanthronepigments, flavanthrone pigments, perylene pigments, diketopyrrolopyrrolepigments, perinone pigments, quinophthalone pigments, anthraquinonepigments, thioindigo pigments, benzimidazolone pigments, isoindolinonepigments, azomethine pigments, and azo pigments.

Specific examples of pigments usable as a colorant are the following(herein is C.I. an abbreviation for Color Index; by a Blue coloredpigment it is understood a pigment that appears blue to the human eye;the other colored pigments are to be understood in an analogous way):

Blue colored pigments which include C.I. Pigment Blue 1, C.I. PigmentBlue 2, C.I. Pigment Blue 3, C.I. Pigment Blue 15:3, C.I. Pigment Blue15:4, C.I. Pigment Blue 15:34, C.I. Pigment Blue 16, C.I. Pigment Blue22, C.I. Pigment Blue 60 and the like; and C.I. Vat Blue 4, C.I. VatBlue 60 and the like;

Red colored pigments which include C.I. Pigment Red 5, C.I. Pigment Red7, C.I. Pigment Red 12, C.I. Pigment Red 48 (Ca), C.I. Pigment Red 48(Mn), C.I. Pigment Red 57 (Ca), C.I. Pigment Red 57:1, C.I. Pigment Red112, C.I. Pigment Red 122, C.I. Pigment Red 123, C.I. Pigment Red 168,C.I. Pigment Red 184, C.I. Pigment Red 202, and C.I. Pigment Red 209;

Yellow colored pigments which include C.I. Pigment Yellow 1, C.I.Pigment Yellow 2, C.I. Pigment Yellow 3, C.I. Pigment Yellow 12, C.I.Pigment Yellow 13, C.I. Pigment Yellow 14C, C.I. Pigment Yellow 16, C.I.Pigment Yellow 17, C.I. Pigment Yellow 73, C.I. Pigment Yellow 74, C.I.Pigment Yellow 75, C.I. Pigment Yellow 83, C.I. Pigment Yellow 93, C.I.Pigment Yellow 95, C.I. Pigment Yellow 97, C.I. Pigment Yellow 98, C.I.Pigment Yellow 109, C.I. Pigment Yellow 110, C.I. Pigment Yellow 114,C.I. Pigment Yellow 128, C.I. Pigment Yellow 129, C.I. Pigment Yellow138, C.I. Pigment Yellow 150, C.I. Pigment Yellow 151, C.I. PigmentYellow 154, C.I. Pigment Yellow 155, C.I. Pigment Yellow 180, and C.I.Pigment Yellow 185;

Orange colored pigments include C.I. Pigment Orange 36, C.I. PigmentOrange 43, and a mixture of these pigments.

Green colored pigments include C.I. Pigment Green 7, C.I. Pigment Green36, and a mixture of these pigments;

-   -   Black colored pigments include: those manufactured by Mitsubishi        Chemical Corporation, for example, No. 2300, No. 900, MCF 88,        No. 33, No. 40, No. 45, No. 52, MA 7, MA 8, MA 100, and No. 2200        B; those manufactured by Columbian Carbon Co., Ltd., for        example, Raven 5750, Raven 5250, Raven 5000, Raven 3500, Raven        1255, and Raven 700; those manufactured by Cabot Corporation,        for example, Regal 400 R, Regal 330 R, Regal 660 R, Mogul L,        Monarch 700, Monarch 800, Monarch 880, Monarch 900, Monarch        1000, Monarch 1100, Monarch 1300, and Monarch 1400; and those        manufactured by Degussa, for example, Color Black FW 1, Color        Black FW 2, Color Black FW 2 V, Color Black FW 18, Color Black        FW 200, Color Black S 150, Color Black S 160, Color Black S 170,        Printex 35, Printex U, Printex V, Printex 140 U, Special Black        6, Special Black 5, Special Black 4A, and Special Black 4.

Other types of pigments such as brown pigments, violet pigments,fluorescent pigments, and metallic powder pigments can also be used as acolorant. The pigments may be used alone or as a mixture of two or morepigments as a colorant.

Blue colored pigments, including cyan pigments, are preferred.

The pigments may be used with or without being subjected to a surfacetreatment of the pigment particles. Preferably, the pigments aresubjected to a surface treatment. Methods for surface treatment includemethods of applying a surface coat of resin, methods of applyingsurfactant, and methods of bonding a reactive material (for example, asilane coupling agent, an epoxy compound, polyisocyanate, or the like)to the surface of the pigment. Suitable examples of pigments withsurface treatment are the modified pigments described in WO 02/04210.Specifically, the blue colored modified pigments described in WO02/04210 are preferred as a colorant.

The pigments have a particle size which is preferably less than 10 μm,more preferably less than 5 μm and especially preferably less than 3 μm.The method for dispersing the pigments may be any known dispersionmethod which is used for the production of ink or toner or the like.Dispersing machines include an ultrasonic disperser, a sand mill, anattritor, a pearl mill, a super mill, a ball mill, an impeller, adispenser, a KD mill, a colloid mill, a dynatron, a three-roll mill, anda press kneader. Details thereof are described in “Latest PigmentApplied Technology” (CMC Publications, published in 1986).

A dispersing agent may be omitted in the preparation of dispersions ofso-called self-dispersing pigments. Specific examples of self-dispersingpigments are pigments with are subjected to a surface treatment in sucha way the pigment surface is compatible with the dispersing liquid.Typical examples of self-dispersing pigments in an aqueous medium arepigments which have ionic or ionizable groups or polyethyleneoxidechains coupled to the particle-surface. Examples of ionic or ionizablegroups are acid groups or salts thereof such as carboxylic acid group,sulphonic acid, phosphoric acid or phosphonic acid and alkali metalsalts of these acids. Suitable examples of self-dispersing pigments aredescribed in WO 02/04210 and these are preferred in the presentinvention. The blue colored self-dispersing pigments in WO 02/04210 arepreferred.

Typically, the amount of pigment in the coating may be in the range ofabout 0.005 g/m² to 2 g/m², preferably about 0.007 g/m² to 0.5 g/m²,more preferably about 0.01 g/m² to 0.2 g/m², most preferably about 0.01g/m² to 0.1 g/m².

The colorant can also be a dye. Any known dyes, such as commerciallyavailable dyes or dyes described in, for example, “Dye Handbook” (editedby the Organic Synthetic Chemistry Association, published in 1970) whichare colored for the human eye, can be used as a colorant in thephotopolymerizable coating. Specific examples thereof include azo dyes,metal complex salt azo dyes, pyrazolone azo dyes, anthraquinone dyes,phthalacyanine dyes, carbionium dyes, quinonimine dyes, methine dyes,and the like. Phthalocyanine dyes are preferred. Suitable dyes aresalt-forming organic dyes and may be selected from oil-soluble dyes andbasic dyes. Specific examples thereof are (herein is CI an abbreviationfor Color Index): Oil Yellow 101, Oil Yellow 103, Oil Pink 312, OilGreen BG, Oil Bue GOS, Oil Blue 603, Oil Black BY, Oil Black BS, OilBlack T-505, Victoria Pure Blue, Crystal Violet (CI42555), Methyl Violet(CI42535), Ethyl Violet, Rhodamine B (CI415170B), Malachite Green(CI42000), Methylene Blue (CI52015). Also, the dyes disclosed in GB 2192 729 may be used as a colorant.

Typically, the amount of dye in the coating may be in the range of about0.005 g/m² to 2 g/m², preferably about 0.007 g/m² to 0.5 g/m², morepreferably about 0.01 g/m² to 0.2 g/m², most preferably about 0.01 g/m²to 0.1 g/m².

Printing-Out Agent

The photopolymerizable layer or another layer of the coating may alsoinclude a printing-out agent, i.e., a compound which is capable ofchanging the color of the coating upon exposure. After image-wiseexposing of the precursor, a visible image can be produced, hereinafteralso referred to as “print-out image”. The printing-out agent may be acompound as described in EP-A-1 491 356 paragraphs [0116] to [0119] onpage 19 and 20, and in U.S. 2005/8971 paragraphs [0168] to [0172] onpage 17. Preferred printing-out agents are the compounds described in WO2006/005688, from line 1 page 9 to line 27 page 20. More preferred arethe IR-dyes as described in the unpublished Patent Application EP 05 105440.1, filed on 21 Jun. 2005, from line 32 page 5 to line 9 page 32.

The Contrast

The contrast of the image formed after image-wise exposure andprocessing with a gum solution is defined as the difference between theoptical density at the exposed area to the optical density at thenon-exposed area, and this contrast is preferably as high as possible.This enables the end-user to establish immediately whether or not theprecursor has already been exposed and processed with a gum solution, todistinguish the different color selections, and to inspect the qualityof the image on the treated plate precursor.

The contrast increases with increasing optical density in the exposedarea and/or decreasing optical density in the non-exposed areas. Theoptical density in the exposed area may increase with the amount andextinction coefficient of the colorant remaining in the exposed areasand the intensity of color formed by the printing-out agent. In thenon-exposed areas, it is preferred that the amount of colorant is as lowas possible and that the intensity of the color print-out agent is aslow as possible. The optical density can be measured in reflectance byan optical densitometer, equipped with several filters (e.g., cyan,magenta, yellow). The difference in optical density at the exposed areaand the non-exposed area preferably has a value of at least 0.3, morepreferably at least 0.4, most preferably at least 0.5. There is nospecific upper limit for the contrast value, but typically the contrastis not higher than 3.0 or even not higher than 2.0. In order to obtain agood visual contrast for a human observer the type of color of thecolorant may also be important. Preferred colors for the colorant arecyan or blue colors, i.e., by blue color it is understood a color thatappears blue to the human eye.

The Top Layer

The coating may include a top layer which acts as an oxygen barrierlayer, hereinafter also referred to as “overcoat layer” or “overcoat”.Preferred binders which can be used in the top layer are polyvinylalcohol and the polymers disclosed in WO 2005/029190, U.S. Pat. No.6,410,205, and EP 1 288 720, including the cited references in thesepatents and patent applications. The most preferred binder for the toplayer is polyvinylalcohol. The polyvinylalcohol preferably has ahydrolysis degree ranging between 74 mol % and 99 mol %. The weightaverage molecular weight of the polyvinylalcohol can be measured by theviscosity of an aqueous solution, 4% by weight, at 20° C. as defined inDIN 53 015, and this viscosity number ranges preferably between 3 and26, more preferably between 3 and 15, most preferably between 3 and 10.

The coating thickness of the top layer is preferably between 0.25 and1.75 g/m², more preferably between 0.25 and 1.3 g/m², most preferablybetween 0.25 and 1.0 g/m². In a more preferred embodiment of the presentinvention, the top layer has a coating thickness between 0.25 and 1.75g/m² and includes a polyvinylalcohol having a hydrolysis degree rangingbetween 74 mol % and 99 mol % and a viscosity number as defined aboveranging between 3 and 26.

In a preferred embodiment, the composition and the thickness of the toplayer are optimized in order to obtain a high sensitivity, good daylightstability, and less or no sludge formation during processing. In orderto reduce sludge, the top layer includes less polyvinylalcohol andpolyvinylalcohol with a lower molecular weight, preferably a viscositynumber of less than 26, more preferably less than 10 is used and athickness as low as possible but more than 0.25 g/m². In order toimprove the sensitivity, a good oxygen barrier is desired, using apolyvinylalcohol with a high hydrolysis degree, preferably 88-98%, and abigger thickness of the top layer. In order to improve the daylightstability, small penetration of oxygen is desired by a using an oxygenbarrier with a reduced barrier property for oxygen, preferably by usinga smaller thickness of the top layer and with polyvinylalcohol having alower hydrolysis degree. Due to a good balance of these elements, anoptimized property for the precursor can be obtained.

The top layer may also include a component selected from the compoundsof the gum solution as described above.

Exposure

The image-wise exposing step is carried out off-press in a plate setter,i.e., an exposure apparatus suitable for image-wise exposing theprecursor by a laser such as a laser diode, emitting around 830 nm, aNdYAG laser, emitting around 1060 nm, a violet laser, emitting around400 nm, or a gas laser such as Ar laser, or by a digital modulatedUV-exposure, e.g., by digital mirror devices, or by a conventionalexposure in contact with a mask. In a preferred embodiment of thepresent invention, the precursor is image-wise exposed by a laseremitting IR-light or violet light.

Pre-Heating

After this image-wise exposing step, the precursor is heated in apre-heating unit to enhance or to speed-up the polymerization and/orcrosslinking reaction. This pre-heat step is carried out within a timeperiod of less than 10 minutes, preferably less than 5 minutes, morepreferably less than 1 minute, most preferably the pre-heat is carriedout immediately after the image-wise exposing, i.e., within less than 30seconds. There is no time limit before the heating may start, but theprecursor is heated as soon as possible after exposing, usually after afew seconds to transport the plate to the pre-heating unit and start theheating process. In this heating step, the precursor is heated at atemperature of preferably 80° C. to 150° C. and for a dwell time ofpreferably 5 seconds to 1 minute. The pre-heating unit is preferablyprovided with heating elements such as IR-lamps, UV-lamps, heated air, aheated metal roll, etc.

Washing

After the pre-heating step, and before the gum-developing step, theprecursor may be washed in a pre-washing station, whereby at least aportion of the top layer can be removed by supplying a wash liquid,i.e., water or an aqueous solution, to the coating of the precursor. Thewashing liquid is preferably water, more preferably tap water.

The term aqueous includes water or mixtures of water with water-miscibleorganic solvents such as alcohols, e.g., methanol, ethanol, 2-propanol,butanol, iso-amyl alcohol, octanol, cetyl alcohol, etc.; glycols, e.g.,ethylene glycol; glycerine; N-methylpyrrolidone; methoxypropanol; andketones, e.g., 2-ptopanone and 2-butanone; etc. The water-miscibleorganic solvent may be present in these mixtures of at most 50% byweight, preferably less than 20% by weight, more preferably less than10% by weight, most preferably no organic solvent is present in theaqueous solution. The aqueous solution may further include a compoundsolubilized or dispersed in water or a mixture of water and awater-miscible solvent. Such compounds may be selected from thecompounds of the gum solution as described above.

The wash liquid used in this step preferably has a temperature rangingbetween 15° C. and 85° C., more preferably between 18° C. and 65° C.,most preferably between 20° C. and 55° C.

The pre-washing station may include at least one pre-washing unitwherein the wash liquid is applied to the precursor by a spraying,jetting, dipping, or coating technique, including spin coating, rollcoating, slot coating, or gravure coating, or by rubbing in with animpregnated pad or by pouring-in, either by hand or in an automaticapparatus. The spraying, jetting, dipping, or coating techniques arepreferred.

An example of a spray nozzle which can be used in the sprayingtechnique, is an air assisted spray nozzle of the type SUJ1,commercially available at Spraying Systems Belgium, Brussels. The spraynozzle may be mounted at a distance of 50 mm to 200 mm between thenozzle and receiving substrate. The flow rate of the spray solution maybe set to 7 ml/min. During the spray process an air pressure in therange of 4.80×10⁵ Pa may be used on the spray head. This layer may bedried during the spraying process and/or after the spraying process.Typical examples of jet nozzles which can be used in the jettingtechnique, are ink-jet nozzles and valve-jet nozzles.

At least one of the pre-washing units may be provided with at least oneroller for rubbing and/or brushing the coating while applying the washliquid to the coating.

The wash liquid used in the pre-washing step can be collected in a tankand the washing liquid can be used several times. The wash liquid can bereplenished by adding fresh water and/or fresh aqueous solution to thetank of the pre-washing unit. The fresh water and fresh aqueous solutionare respectively water and aqueous solution which have not been usedbefore for washing a precursor. In an alternative way, the wash liquidmay be used once-only, i.e., only fresh water or fresh aqueous solutionis applied to the coating by preferably a spraying or jetting technique.Preferably tap water is used in this alternative way.

The pre-washing station may include two or more pre-washing units,preferably two or three pre-washing units.

In a preferred embodiment of the present invention, the pre-washingstation includes a first and a second pre-washing unit whereby theprecursor is firstly washed in the first pre-washing unit andsubsequently washed in the second pre-washing unit. The precursor can befirstly washed in the first pre-washing unit with washing liquid whichhas been used in the second pre-washing unit, and, subsequently, washedin the second pre-washing unit with fresh water or fresh aqueoussolution by preferably a spraying or jetting technique. In analternative way, the first and second pre-washing units preferably havethe configuration of a cascade system, whereby the wash liquid used forwashing the precursor in the first and second pre-washing unit arerespectively present in a first and a second tank, and whereby the washliquid of the second tank overflows to the first tank when fresh wateror fresh aqueous solution is added in the second pre-washing unit.

In another preferred embodiment of the present invention, thepre-washing station may include a first, a second, and a thirdpre-washing unit whereby the precursor is firstly washed in the firstpre-washing unit, subsequently in the second pre-washing unit, andfinally in the third pre-washing unit. The precursor can be firstlywashed in the first pre-washing unit with washing liquid which has beenused in the second pre-washing unit, subsequently washed in the secondpre-washing unit with washing liquid which has been used in the thirdpre-washing unit, and finally washed in the third pre-washing unit withfresh water or fresh aqueous solution by preferably a spraying orjetting technique. In an alternative way, the first, second, and thirdpre-washing units preferably have the configuration of a cascade system,whereby the wash liquids used for washing the precursor in the first,second, and third pre-washing units are respectively present in a first,a second, and a third tank, and whereby the wash liquid of the thirdtank overflows to the second tank when fresh water or fresh aqueoussolution is added in the third pre-washing unit and the wash liquid ofthe second tank overflows to the first tank.

In another preferred embodiment of the present invention, the washliquid used in each of the pre-washing units may be also regenerated byremoving insoluble material present in the wash liquid. The presence ofinsoluble material in the wash liquid may be caused by several reasons,e.g., by washing of a pigment containing coating, by evaporation ofsolvent or water of the washing liquid, or by sedimentation,coagulation, or flocculation of components in the wash liquid. Theinsoluble material can be removed by several techniques such asfiltration, ultra-filtration, centrifugation, or decantation. A suitableapparatus for disposing a waste solution such as the wash liquid of thepreferred embodiments of the present invention is described in EP-A 747773. The apparatus can be connected to the tank of a pre-washing unit toregenerate the used wash liquid by circulation of the wash liquid over afilter or a filter membrane. The wash liquid can be circulated over thefilter or filter membrane continuously, periodically, or during thewashing time, or the circulation is regulated by the measurement of theturbidity or transparency (i.e., optical transmission) of the washliquid whereby the circulation starts when the turbidity exceeds anupper value and stops when an under value is reached. The upper andunder turbidity value can be chosen in relation to the desired degree ofpurification, generally the optical transmission of the wash liquid isnot lower than 50% of its value at starting, preferably not lower than80%, more preferably not lower than 95%.

In the present invention, at least a portion of the top layer can beremoved in the washing step, preferably more than 50% of the top layeris removed, more preferably more than 80%, most preferably more than95%. Preferably, the photopolymerizable layer is substantially notextracted or solubilized in the washing step whereby the wash liquidused in the washing step does not contain components of thephotopolymerizable layer in a concentration of less than 2% by weightof, more preferably less than 1% by weight of these ingredients of thepotopolymerizable layer, most preferably less than 0.5% by weight. Thecomponents of the photopolymerizable layer which are preferably as muchas possible omitted in the wash liquid are polymerizable monomer,multifunctional monomer, initiator, inhibitor, and/or sensitizer.

Gum-Processing

After the heating step in the pre-heating unit or, when a washing stepis present, after the washing step, the precursor is developed in agumming station by applying a gum solution to the coating of theprecursor, thereby removing the non-exposed areas of thephotopolymerizable layer from the support and gumming the plate in asingle step. The gumming station includes at least one gumming unitwherein the gum is applied to the precursor by a spraying, jetting,dipping, or coating technique or by rubbing in with an impregnated pador by pouring-in, either by hand or in an automatic apparatus.

An example of a spray nozzle which can be used in the sprayingtechnique, is an air assisted spray nozzle of the type SUJ1,commercially available at Spraying Systems Belgium, Brussels. The spraynozzle may be mounted at a distance of 50 mm to 200 mm between thenozzle and receiving substrate. The flow rate of the spray solution maybe set to 7 ml/min. During the spray process, an air pressure in therange of 4.80×10⁵ Pa may be used on the spray head. This layer may bedried during the spraying process and/or after the spraying process.Typical examples of jet nozzles which can be used in the jettingtechnique, are ink-jet nozzles and valve-jet nozzles.

At least one of the gumming units may be provided with at least oneroller for rubbing and/or brushing the coating while applying the gum tothe coating. The gum used in the developing step can be collected in atank and the gum can be used several times. The gum can be replenishedby adding a replenishing solution to the tank of the gumming unit. In analternative way, the gum solution may be used once-only, i.e., onlystarting gum solution is applied to the coating by preferably a sprayingor jetting technique. The starting gum solution is a gum solution whichhas not been used before for developing a precursor and has the samecomposition as the gum solution used at the start of the development.

The replenishing solution is a solution which may be selected from astarting gum solution, a concentrated gum solution, a diluted gumsolution, a solution of a non-ionic surfactant, water, a solution of abuffer having a pH ranging between 4 and 7 or a baking gum. Aconcentrated or diluted gum solution is a solution including a higheror, respectively, lower concentration of gum additives as defined above.A concentrated gum solution can be added as replenishing solution whenthe concentration of active products is under a desired level in the gumsolution. A diluted gum solution or water can be used when theconcentration of active products is above a desired level in the gumsolution or when the viscosity of the gum solution is increased or whenthe volume of the gum solution is under a desired level, e.g., due toevaporation of the solvent or water. A solution of a non-ionicsurfactant or a solution of a buffer can be added when the gum solutionneeds a higher concentration of a surfactant or when the pH of the gumsolution needs to be controlled at a desired pH value or at a desired pHvalue in a range of two pH values, e.g., between 4 and 7.

The addition of replenishing solution, i.e., the type and the amount ofreplenishing solution, may be regulated by the measurement of at leastone of the following parameters such as the number and area of plateprecursors developed, the time period of developing, the volume in eachgumming unit (minimum and maximum level), the viscosity (or viscosityincrease) of the gum solution, the pH (or pH change) of the gumsolution, the density (or density increase) of the gum solution and theconductivity (or conductivity increase) of the gum solution, or acombination of at least two of them. The density (or density increase)of the gum solution can be measured with a PAAR density meter.

The gum solution used in this step preferably has a temperature rangingbetween 15° C. and 85° C., more preferably between 18° C. and 65° C.,most preferably between 20° C. and 55° C.

In a preferred embodiment of the present invention, the gumming stationincludes a first and a second gumming unit whereby the precursor isfirstly developed in the first gumming unit and subsequently developedin the second gumming unit. The precursor may be firstly developed inthe first gumming unit with gum solution which has been used in thesecond gumming unit, and, subsequently, developed in the second gummingunit with a starting gum solution by preferably a spraying or jettingtechnique. In an alternative way, the first and second gumming unitpreferably have the configuration of a cascade system, whereby the gumsolution used for developing the precursor in the first and secondgumming unit are respectively present in a first and a second tank, andwhereby the gum solution of the second tank overflows to the first tankwhen replenishing solution is added in the second gumming unit.Optionally, also to the first gumming unit a replenishing solution canbe added and this replenishing solution may be the same or anotherreplenishing solution than added to the second gumming unit, e.g., adiluted gum solution, a solution of a non-ionic surfactant, or water canbe added as replenisher to the first gumming unit.

In another preferred embodiment of the present invention, the gummingstation includes a first, a second, and a third gumming unit whereby theprecursor is firstly developed in the first gumming unit, subsequentlyin the second gumming unit, and finally in the third gumming unit. Theprecursor may be firstly developed in the first gumming unit with gumsolution which has been used in the second gumming unit, subsequentlydeveloped in the second gumming unit with gum solution which has beenused in the third gumming unit, and finally developed in the thirdgumming unit with starting gum solution by preferably a spraying orjetting technique. In an alternative way, the first, second, and thirdgumming units preferably have the configuration of a cascade system,whereby the gum solution used for developing the precursor in the first,second, and third gumming unit are respectively present in a first, asecond, and a third tank, and whereby the gum solution of the third tankoverflows to the second tank when replenishing solution is added in thethird gumming unit, and whereby the gum solution of the second tankoverflows to the first tank. Optionally, also to the second and/or firstgumming unit(s) a replenishing solution may be added and thisreplenishing solution may be the same or another replenishing solutionthan added to the third gumming unit, e.g., a diluted gum solution, asolution of a non-ionic surfactant, or water can be added as replenisherto the second or first gumming unit. In another option, two differentreplenishing solutions can also be added to one gumming unit, e.g., astarting gum solution and water.

In another preferred embodiment of the present invention, the gumsolution used in each of the gumming units may be regenerated byremoving an insoluble material present in the gum solution of a gummingunit. The presence of insoluble material in the gum solution may becaused by several reasons, e.g., by developing a pigment containingcoating, by evaporation of solvent or water of the gum solution, or bysedimentation, coagulation, or flocculation of components in the gumsolution. The insoluble material can be removed continuously or in batchform by several techniques such as filtration, ultra-filtration,centrifugation, or decantation. A suitable apparatus for disposing awaste developing solution such as the gum solution of the preferredembodiments of the present invention is described in EP-A 747 773. Theapparatus can be connected to the tank of a gumming unit to regeneratethe used gum solution by circulation of the gum solution over a filteror a filter membrane. The gum solution can be circulated over the filteror filter membrane continuously, periodically or during the developmenttime, or the circulation is regulated by the measurement of theturbidity or transparency (i.e., optical transmission) of the gumsolution whereby the circulation starts when the turbidity exceeds anupper value and stops when an under value is reached. The upper andunder turbidity value can be chosen in relation to the desired degree ofpurification, generally the optical transmission of the gum solution isnot lower than 50% of its value at starting, preferably not lower than80%, more preferably not lower than 95%.

Drying

According to another preferred embodiment of the present invention, theplate can be dried after the gum-processing step in a drying unit. In apreferred embodiment, the plate is dried by heating the plate in thedrying unit which may contain at least one heating element selected froman IR-lamp, a UV-lamp, a heated metal roller or heated air. In apreferred embodiment of the present invention, the plate is dried withheated air as known in the drying section of a conventional developingmachine.

Baking

According to another preferred embodiment of the present invention, theplate can be heated in a baking unit, optionally after drying the plate.In a preferred embodiment of the present invention, when the plate isheated in a baking unit, the precursor is developed by using a bakinggum and the gum solution is preferably replenished by adding areplenishing baking gum. The replenishing baking gum is a solution whichmay be selected from a starting baking gum, i.e., a solution having thesame composition as the baking gum used at the start of the development,a concentrated baking gum, or a diluted baking gum, i.e., a solutionhaving a higher or, respectively, lower concentration of additives thanthe starting baking gum, and water.

The baking unit may contain at least one heating element selected froman IR-lamp, a UV-lamp, a heated metal roller, or heated air. The plateis preferably heated in the baking unit at a temperature above 150° C.and less than the decomposition temperature of the coating, morepreferably between 200° C. and 295° C., most preferably between 250° C.and 290° C. A longer heating time is usually used when a lower heatingtemperature is used, and a shorter heating time is used when a higherheating temperature is used. The plate is preferably heated over a timeperiod of less than 10 minutes, more preferably less than 5 minutes,most preferably less than 2 minutes.

In a preferred embodiment of the present invention, the plate is heatedby the method as described in EP-A 1 506 854. In another preferredembodiment of the present invention, the plate is heated by the methodas described in WO 2005/015318.

In another preferred embodiment of the present invention, the dryingstep and the heating step may be combined in one single step wherein theplate, after the gum-developing step, is dried and heated in anintegrated drying-baking station.

Single Apparatus

According to another preferred embodiment of the present invention, thepre-heating unit and the gumming unit are coupled together by amechanical plate conveying device. The gumming unit may be furthercoupled by a mechanical plate conveying device to the drying unit. Thedrying station may be further coupled by a mechanical plate conveyingdevice to the baking unit. The gumming unit may also be further coupledby s mechanical plate conveying device to the integrated drying-bakingunit.

According to still another preferred embodiment of the presentinvention, the pre-heating unit, coupled to the gumming unit, may befurther coupled to the plate setter by a mechanical plate conveyingdevice wherein the precursor is shielded from ambient light.

EXAMPLES Preparation of Aluminum Support S-1

A 0.3 mm thick aluminum foil was degreased by spraying with an aqueoussolution containing 26 g/l of NaOH at 65° C. for 2 seconds and rinsedwith demineralized water for 1.5 seconds. The foil was thenelectrochemically grained for 10 seconds using an alternating current inan aqueous solution containing 15 g/l of HCl, 15 g/l of SO₄ ²⁻ ions and5 g/l of Al³⁺ ions at a temperature of 37° C. and a current density ofabout 100 A/dm². The aluminum foil was then desmutted by etching with anaqueous solution containing 5.5 g/l of NaOH at 36° C. for 2 seconds andrinsed with demineralized water for 2 seconds. The foil was subsequentlysubjected to anodic oxidation for 15 seconds in an aqueous solutioncontaining 145 g/l of sulfuric acid at a temperature of 50° C. and acurrent density of 17 A/dm², then washed with demineralized water for 11seconds and post-treated for 3 seconds by spraying a solution containing2.2 g/l of polyvinylphosphonic acid at 70° C., rinsed with demineralizedwater for 1 second and dried at 120° C. for 5 seconds.

The support thus obtained was characterized by a surface roughness Ra of0.35-0.4 μm, measured with interferometer NT1100, and had an anodicweight of 3.0 g/m².

Preparation of Photosensitive Layer P-1:

The coating compositions for the photosensitive layer P-1 were preparedby mixing the ingredients as specified in Table 2. The resultingsolution was coated on a support. After coating, the plate was dried for1 minute at 120° C. in a circulation oven. The resulting applied amountis 1.20 g/m².

Table 2: Compositions of the Photosensitive Layer Solutions

TABLE 2 Compositions of the Photosensitive Layer Solutions COMPOSITION/INGREDIENTS P-1 Koma 30 (1) 138.20 (g) FST 426R (2) 8.82 (g) Mono Z1620(3) 93.13 (g) Heliogene Blue D7490 (4) 55.97 (g) DISB (5) 2.92 (g) HABI(6) 4.42 (g) MBT (7) 0.20 (g) Hostanox 03 (8) 0.34 (g) Edaplan LA411 (9)0.68 (g) Dowanol PM (10) 526.39 (g) Butanone 168.92 (g) (1) Koma 30 is acopolymer of vinyl butyral, vinylalcohol and vinylacetate, esterifiedwith trimellit acid, 13.9% by weight, commercially available fromClariant. (2) FST 426R is a solution in 2-butanone containing 88.2% byweight of a reaction product from 1 mole of2,2,4-trimethylhexamethylenediisocyanate and 2 moles ofhydroxyethylmethacrylate (viscosity 3.30 mm²/s at 25° C.). (3) MonoZ1620 is a solution in 2-butanone containing 30.1% by weight of areaction product from 1 mole of hexamethylenediisocyanate, 1 mole of2-hydroxyethylmethacrylate and 0.5 mole of 2-(2-hydroxyethyl)-piperidine(viscosity 1.7 mm²/s at 25° C.). (4) Heliogene Blue D7490 dispersion(9.9% by weight, viscosity 7.0 mm²/s at 25° C.), trade name of BASF, asdefined in EP 1 072 956 (5) DISB is 1,4-di[3,5-dimethoxy,4-isobuthoxy-styryl]benzene. (6) HABI is2,2′-bis(2-chlorophenyl)-4,4′,5,5′-tetraphenyl-1,2-bisimidazole. (7) MBTis 2-mercaptobenzothiazole. (8) Hostanox 03 is a phenolic antioxidant,commercially available from Clariant. (9) Edaplan LA411 is a surfactant(solution of 10% by weight in Dowanol PM ® trade mark of Dow ChemicalCompany) obtained from Munzing Chemie. (10) Dowanol PM is propyleneglycol monomethylether, trade mark of Dow Chemical Company.

Preparation of Overcoat Layer OC-1:

On top of the photosensitive layer a solution in water with thecomposition as defined in Table 3 was coated and was dried at 110° C.for 2 minutes. The so-formed protective overcoat OC-1 has a drythickness of 1 g/m².

TABLE 3 Composition of Overcoat Solution COMPONENT OC-1 partiallyhydrolyzed polyvinylalcohol (degree 9.73 of hydrolysis 88%, viscosity 4mPa · s in a solution of 4 wt. % at 20° C.) (g) partially hydrolyzedpolyvinylalcohol (degree 4.25 of hydrolysis 88%, viscosity 8 mPa · s ina solution of 4 wt. % at 20° C.) (g) fully hydrolyzed polyvinylalcohol(degree of 8.50 hydrolysis 98%, viscosity 6 mPa · s in a solution of 4wt. % at 20° C.) (g) Acticide LA1206 (1) 0.05 (g) Lupasol P (2) 0.23 (g)Lutensol A8 (3) 0.22 (g) Water 977.03 (g) (1) Acticide LA1206 is abiocide, commercially available from Thor. (2) Lupasol P is a solutionof 50% by weight of a polyethylene imine in water, commerciallyavailable from BASF. (3) Lutensol A8 (90% by weight) is a surface activeagent, commercially available from BASF.

Preparation of the Printing Plate

The precursor was imaged with an energy of 25 μJ/cm² in a violet platesetter device Advantage DL3850 (Addressability: 1270 dpi).

A pre-heat was carried on in the pre-heat section of a VSP-85 (travelspeed=1.2 m/min., plate temperature=110° C.). The time between the endof the exposure and the start of the pre-heat treatment was varied: 0.5,1, 5, 10, 15, 30, 60 minutes.

After the pre-heat, the printing plates were developed in the developingsection of a VSP-85 in a Gum-1 solution at 25° C. at 1.2 m/min.)

Gum-1 is a solution prepared as follow:

-   -   To 750 g demineralized water    -   100 ml of Dowfax 3B2 (commercially available from Dow Chemical)    -   31.25 g 1,3-benzene disulphonic acid disodium salt (available        from Riedel de Haan)    -   31.25 ml Versa TL77 (a polystyrene sulphonic acid available from        Alco Chemical)    -   10.4 g trisodium citrate dihydrate,    -   2 ml of Acticide LA1206 (a biocide from Thor),    -   2.08 g of Polyox WSRN-750 (available from Union Carbide) were        added under stirring and demineralized water was further added        to 1000 g.    -   pH is between 7.2 and 7.8

The printing plates were evaluated by inspecting the highlight renderingof a 110 lpi raster. In Table 4 the minimum % dots which were perfectlyor almost perfectly visible on the printing plate are shown as afunction of time between end of exposure and start of pre-heat (A time(min.)).

TABLE 4 Highlight Rendering as Function of Δ Time Δ time Highlight(min.) rendering 110 lpi 0.5 3% 1 3% 5 3% 10 4% 15 5% 30 6% 60 6%

From Table 4 it is clear that an increasing time between the end of theexposure and the start of the pre-heat (Δ time) results in a decrease ofthe highlight rendering. When Δ time is between 0.5 and 5 minutes a 3%dot pattern is still visible on the printing plate after processing.When Δ time amounts to 30 or 60 minutes the 3% dot pattern (even the 4and 5% dot patterns) are no longer visible.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

1-13. (canceled) 14: A method of making a lithographic printing platecomprising the steps of: a) providing a lithographic printing plateprecursor including a support having a hydrophilic surface or which isprovided with a hydrophilic layer, a coating on the support including aphotopolymerizable layer and, optionally, an intermediate layer betweenthe photopolymerizable layer and the support, wherein thephotopolymerizable layer includes a polymerizable compound, apolymerization initiator, and a binder; b) image-wise exposing thecoating by a laser in a plate setter; c) heating the precursor in apreheating unit within a time period of less than 10 minutes after step(b); and d) treating the precursor in a gumming station, including atleast one gumming unit, whereby a gum solution is applied to theprecursor, thereby removing non-exposed areas of the photopolymerizablelayer from the support and gumming the plate in a single step; whereinthe gum solution has a pH-value ranging between 3 and
 9. 15: A methodaccording to claim 14, wherein the time period is less than 5 minutes.16: A method according to claim 14, wherein the time period is less than1 minute. 17: A method according to claim 14, wherein in step (c) theprecursor is heated in the preheating unit at a temperature rangingbetween 80° C. and 150° C. 18: A method according to claim 14, whereinin step (c) the precursor is heated in the preheating unit for a dwelltime ranging between 5 seconds to 1 minute. 19: A method according toclaim 14, wherein the preheating unit is provided with a heatingelement. 20: A method according to claim 19, wherein the heating elementis an IR-lamp, a UV-lamp, heated air, or a heated metal roll. 21: Amethod according to claim 14, wherein the preheating unit is coupled tothe gumming unit by a mechanical plate conveying device. 22: A methodaccording to claim 14, wherein the preheating unit is coupled to theplate setter by a mechanical plate conveying device wherein theprecursor is shielded from ambient light. 23: A method according toclaim 14, wherein the preheating unit is coupled to the gumming unit bya mechanical plate conveying device and wherein the preheating unit isfurther coupled to the plate setter by a mechanical plate conveyingdevice wherein the precursor is shielded from ambient light. 24: Amethod according to claim 14, wherein the laser emits violet light. 25:A method according to claim 14, wherein the laser emits infrared light.